The task of exchange installations is not only to switch through a connection for making possible an interchange of information between two terminal stations, but also during the setting-up phase of a connection which is originated automatically by the subscriber, information to the subscriber is necessary to tell him about the state of his connection. On each connection, the first information needed about the readiness for dialing of the exchange is transmitted to the subscriber as dial-tone on lifting the handset. The dial-tone mostly consists of a standardized frequency which is sent permanently or in a determined sequence.
When a subscriber has finished his dialing procedure completely, a ringing or busy tone is sent to him after checking the possible connection paths and the condition of the wanted terminal. There would be ringing the wanted subscriber but the busy-tone interval is clearly different, for example, a speedy sequence of the standardized frequency.
Besides the audio-tones, there are further possibilities to inform a calling subscriber. For example, it is useful to give a special indication to a subscriber who has dialed to a non-existent connection path or terminal. In this case the normal busy-tone is not sufficient, because the subscriber will keep on trying to get the wanted connection. Such an indication information may, for example, consist of a special tone sequence and a following announcement: "No connection possible when dialing this number".
In many cases it is further desired to send common valid information to the subscribers. These special announcement services can comprise, for example, time announcement, weather forecast, news, etc.
In most exchanges the audio-tones, indication information and announcements are switched through via the common network that is used for speech connections. The consequence of this is that while switching on special signals of the described kind, the coupling paths are not available for real connections. Therefore an existing coupling network is less efficient for real connections.
To avoid this disadvantage it was proposed for PCM-exchanges to store tone signals digitally and to send them in this form directly to terminal sets. Such an arrangement is described in the German patent application DT-OS No. 19 45 155. In the German patent DT-PS No. 23 14 272 is described how the sequence of an audio-tone can be included also in the digital form. These arrangements are disadvantageous as they are only applicable for tone frequency signals, because the storage expenses for announcement texts in the digital form would increase enormously.
The instant invention provides a circuit arrangement which is applicable for all kinds of special signals and special information and is also able to receive the dialing and special information signals coming from the terminals.
The advantage of the invention is that for all signals which are to be interchanged between the exchange and the terminals there is a bus system available whereby the loading of the remaining coupling network by such connections is totally avoided.
Owing to the fixed coordination of time channels to the different special signals, an exchange can be fitted easily with several kinds of signals according to the case of application. As all controlling functions for the special coupling network are resolved by a peripheral control unit, for all these connections no additional expenses are necessary. All connections to tone receivers, tone senders and announcement text senders are kept away from the remaining connection coupling network that provide real connections. Owing to this clear organization, exchanges are able to be dimensioned favorably more easily. Thus, for example, a special switching network for special signals can be added without making necessary any modifications at the coupling network KV of the installation or at the whole arrangement. With the application of integrated multiplex circuits, the costs for each terminal unit are also then still acceptable.